The present invention relates to vibratory material feeders driven in mechanical resonance.
Vibratory feeders exhibit many advantages over other types of material feeding systems such as belt feeders or motor driven auger feeders. For example, non-vibratory feeders, while exhibiting excellent long-term control over material feeding, often have non-uniform material feeding over the short term, and are often unacceptable for applications such as ingredient mixing which requires that specified material ratios be maintained at all times.
Vibratory feeders are also mechanically simplier than these other types of feeders because they operate without bearings, motor brushes, seals and the like. This results in a higher reliability and lower cost material feeder compared with other types of material feeders, and since vibratory feeders typically have no sparking electrical contacts, they are readily adaptable to highly explosive hazardous environments.
In addition, since the only component of the vibratory material feeder that is in direct contact with the material being fed is a feed tray, or the like, cleaning is greatly simplified. Vibratory feeders also exhibit excellent temperature stability, and are capable of operating with high efficiency; for example, feeding 25 tons of material per hour with only 60 watts electrical input.
However, commercially available vibratory feeders are not without disadvantage. For example, since vibratory feeders depend upon the frequency of mechanical resonance of the feeder, which varies with temperature, feed rates are affected by ambient temperature changes and temperature changes due to warm-up. Feeders driven from the power line exhibit a high degree of sensitivity to line voltage and frequency variation. Also, the material feed rate is often not easily controllable because of the non-linear relationship existing between the actual feed rate and a desired command value, and the fact that the feed rate is not zero based, (i.e., a power greater than zero must be applied to the vibratory feeder before material will begin feeding). This offset changes with type of material and material headload.
Another typical disadvantage of vibratory feeders is that they provide no indication of material feed which makes sensing of clogs and the like very difficult. Also, when materials of different densities are used, or when the feeding tray is changed, tedious mechanical tuning is required in order to reestablish the desired mechanical resonant point corresponding to the drive frequency of the vibratory actuator. Finally, since most feeders are driven with phase control of a voltage derived from a power line, unacceptable electrical interference is generated.
One attempt to control the amount of material fed by a vibrating machine is described in Japanese Patent No. 58-193814 to Muramatsu, wherein the actual oscillation frequency and amplitude of a vibrating unit are detected by separate detectors and the actual quantity of material conveyed by the machine is calculated therefrom. This is done simply by multiplying the detected vibrating frequency by the detected vibrating amplitude and comparing the value thus calculated with a similar value calculated for a quantity desired to be conveyed. The difference between the values is equalized by adjusting the frequency or amplitude of oscillation. However, such a system does not provide for control of the displacement amplitude and frequency independently of each other.
It has also been recognized that maximum efficiency occurs when a vibratory feeder is driven at the natural mechanical frequency of the system. However, conventional vibratory feeder systems are typically driven at a frequency different from the natural frequency because large and uncontrollable vibrations may occur unless sufficient damping is provided. For this reason, the conventional system is usually designed so that its natural mechanical frequency is above or below the electrical drive frequency resulting in a corresponding loss of efficiency.
Recently, attempts have been made to intentionally drive vibratory feeders at their natural frequency. However, such feeders have utilized complex control schemes which do not prov de independent control of vibration amplitude and frequency. One resonance type electromagnetic vibrating feeder is described in Japanese Patent No. 58-113014 to Nonaka, wherein the speed of transport of the body of the feeder is controlled while attempting to maintain resonance frequency vibration. Nonaka controls the transport speed by attempting to maintain the value obtained by multiplying the vibrating amplitude by the vibrating frequency (as described in Muramatsu, noted above). The control provided by Nonaka depends on a combined value obtained from amplitude and frequency detectors. More specifically, Nonaka includes a detector for detecting the difference between the frequency of the exciting force and the frequency of the vibrating force, and an amplitude detector. The actual frequency difference is compared to a desired frequency difference and is used by a DC-AC circuit to vary the exciting force frequency. The values from the two detectors are both fed to a multiplier and the value of the multiplication of the amplitude and frequency is then compared to an analogous value for a desired transport speed. The result is used, via an AC-DC circuit, to regulate the DC voltage level and thereby control the actual transport speed of the apparatus. Moreover, Nonaka attempts to maintain resonance by trying to maintain a particular mathematical relationship between the exciting force frequency and the vibrating force frequency.
Further, the Nonaka feeder seeks to maintain a selected speed of motion of the feeder trough and does not seek to maintain a particular amplitude of movement. In fact, the amplitude of movement of the trough of the feeder is freely varied for the purpose of maintaining transport speed. That is, Nonaka does not maintain a selected amplitude of displacement, regardless of transport speed, and independently of the means for maintaining resonance.
Another resonance type electromechanical feeder device is described in Patent Cooperation Treaty Application No. WO 86/02058 (Pross, et al.), which appears to measure the resonance frequency and oscillation amplitude of a mass oscillating system and then alters the excitation voltage of the drive mechanism to cause the actual oscillation frequency to approach, but not match, the resonance frequency of the system. More particularly, an electromagnetic measuring coil and magnet sensor arrangement generates a voltage whose amplitude is proportional to the degree of amplitude of oscillation of a material holding trough, and whose frequency is compared with an excitation frequency. The sensor is used to generate energizing pulses of varying duration, timing and frequency which are applied to the excitation coil of the drive mechanism, resulting in adjustment of the actual frequency and amplitude of oscillation of the feeder.
Other resonance type vibratory feeders seek to control both the amplitude and frequency of feeder vibration using single sensor arrangements. One example of such systems is disclosed in U.S. Pat. No. 4,331,263 to Brown, wherein the amplitude and frequency of vibration are sensed by detecting the third harmonic of the drive current signal.